Field of the Invention
This invention relates to an apparatus for reproducing acoustic signals by a headphone device.
In reproducing acoustic signals using a pair of headphone units mounted on the listener's head and applied in the vicinity of the listener's ears, as in the case of a headphone apparatus for reproducing the acoustic signals by headphone units, there has been known a binaural system as means for optimizing the sense of the direction of a sound image or the sense of the sound source lying at some fixed position outside the listener's head.
With the binaural acoustic signal reproducing system, as disclosed for example in the Japanese Patent Kokoku Publication No. 283/1978, the acoustic signals reproduced by the headphone device are subjected in advance to a predetermined signal processing.
The sense of the direction of the sound image, or the sense of the sound source lying at some fixed position outside of the listener's head, are governed by the difference in sound volume, and time as well as in the phase of the sounds heared by the left and right ears.
The above mentioned signal processing refers to signal processing which, when the acoustic sound is to be reproduced by the speaker units, the acoustic effects equivalent to those produced by the difference in distance from the sound source, that is, the speaker units, placed at some distance from the listener, to the listener's left and right ears, or the reflection or diffraction in the vicinity of the listener's head may be simulated in the acoustic output reproduced by the headphone device. Such signal processing may be realized by subjecting the acoustic signals for the listener's left and right ears to, for example, convolutional integration of the impulse response corresponding to the above mentioned acoustic effects.
Meanwhile, when the acoustic sound is to be reproduced by speaker units placed at a distance from the listener, the absolute position of the sound image is not changed even if the listener has moved his or her body or head, so that the relative direction or position of the sound image felt by the listener is changed. Conversely, when the acoustic sound is reproduced in accordance with the binaural system, using the headphone device, the headphone device is moved with the listener's head when the listener has turned his or her head, so that the relative direction and position of the sound image as sensed by the listener remain unchanged.
In this manner, when the acoustic sound is reproduced in accordance with the binaural system, using the headphone device, the sound field may be formed within the listener's head, on account of the difference in the state of shift of the sound image with respect to the change in the orientation of the listener's head, with the result that the sound image cannot be easily fixed at a position ahead of the listener. In addition, the sound image lying ahead of the listener tends to be moved upwards.
There has also been proposed an acoustic signal reproducing system in which, as described in Japanese Patent Kokai Publication No. 227/1967 or Japanese Patent Kokoku Publication No. 19242/1979, the changes caused in the orientation of the listener's head are sensed, and the signal processing state is changed on the basis of the sensed results so as to provide an optimum forward fixed sound source orientation feeling for the headphone device. With this type of acoustic signal reproducing system, a direction sensor, such as a gyrocompass or magnetic needle, is positioned on the listener's head. The above mentioned level adjustment circuit and the delay circuit, adapted for processing acoustic signals, are controlled on the basis of the results of detection by the direction sensor to provide the ambience of a sound field similar to that provided by sound reproduction by the speaker units placed at some distance from the listener.
With the above described conventional binaural acoustic reproducing system, in which gyrocompass or the like direction sensor is provided in the headphone device, the contents of signal processing for the acoustic signals dependent upon the changes in the direction of the listener's head may be controlled to provide a satisfactory fixed sound image orientation feeling.
However, for controlling the contents of the signal processing for the acoustic signals in dependence upon changes in the listener's head position, it is necessary to measure in advance the impulse response, that is the transmission characteristics, corresponding to the acoustic effects applied to acoustic signals for left and right ears, for each of predetermined angles, to store voluminous transfer characteristic data in storage means and to read out the data responsive to occasional changes in the listener's head position, for performing the necessary real-time convolutional integration of the acoustic signals. A processing apparatus with a large processing capacity and a high processing speed needs to be employed as processing means executing such real time convolutional integration with variable coefficients.